(Patent Reference 1)
JP-A-2006-261008
(Patent Reference 2)
JP-A-2000-285910
(Patent Reference 3)
JP-A-2007-5279
In recent years, electronics has been remarkably developed, which has enabled the production of small lightweight thin portable electronic devices with multiple functions. Accordingly, a demand for small lightweight thin reliable batteries as power source for such portable electronic devices has been rapidly increased. In response to the demand, a multilayer lithium ion secondary battery with a laminate where positive electrode layers and negative electrode layers are stacked via solid electrolyte layers. A multilayer lithium ion secondary battery is fabricated by stacking battery cells having a thickness of tens of μm, so that it can be easily realized to produce small lightweight thin batteries. Especially, parallel type or serial-parallel type multilayer batteries are superior in that they have high discharge capacity even when the battery cell area is small. And all solid state lithium ion secondary batteries using solid electrolyte material in place of electrolyte liquid are reliable without problems such as liquid leakage or liquid evaporation. Furthermore, high unit cell voltage and high energy density can be obtained because they are batteries using lithium.
As for a multilayer all solid state lithium ion secondary battery, a battery is suggested according to Patent Reference 1, where a negative electrode layer, a collector, a negative electrode layer, a electrolyte layer, a positive electrode layer, a collector, and a positive electrode layer are stacked one on top of the other in sequence. FIG. 16 is cross-sectional diagrams of a conventional lithium ion secondary battery. In a battery 101, a collector layer 105 disposed between positive electrode layers 103 and a collector layer 106 disposed between negative layers 104 are stacked interposed by an electrolyte layer 102. The battery shown in FIG. 16 is a parallel type battery where a positive electrode terminal 107 and a negative electrode terminal 108, which are the output terminals of the battery, are disposed along the sides of the laminate and contact with positive electrode layers 103 and negative electrode layers 104, respectively. The battery described in Patent Reference 1 consists of a positive electrode layer and a negative electrode layer made of only active material. Active material such as metal oxide or metal sulfide is described as preferred material for a positive electrode, and active material such as metal lithium or lithium alloy is described, as preferred material for a negative electrode.
As shown in FIG. 16, the distance from an electrode terminal to the edge of an electrode layer (positive electrode and negative electrode) is comparatively long in parallel multilayer batteries. And, among active material used for a lithium ion battery, oxide material has advantage that lithium ion migration causes small volume change of the electrode layer and hardly causes pulverization and abrasion, but at the same time it has disadvantage that its electrical conductivity is low. When electrical resistance in an electrode layer is high, the internal impedance of the battery is high, then discharge characteristic on load degrades, and discharge capacity decreases. Corresponding to these problems, a battery structure is adopted to reduce electrode impedance by stacking a collector layer with high conductivity on an electrode layer with low conductivity in Patent Reference 1.
However, to fabricate a battery with such a structure, coating and drying process is necessary for the formation of an electrode layer, a collector layer, and an electrode layer for each of a positive electrode layer and a negative electrode layer. Furthermore, alignment process is necessary to align an electrode layer and a collector layer. So, especially in the case that many layers are stacked, the manufacturing process is complicated and eventually the production cost is high. Since many coating and drying processes are repeated, resulting battery manufacturing yield will become quite low due to the intensity drop and damage (sheet-attack) of an electrolyte layer which is disposed under an electrode layer or a collector layer caused by solvent used in the formation process of the electrode layer and the electrolyte layer.
Patent Reference 2 describes attempted procedure to reduce the electrical resistivity of an electrode layer itself in a multilayer lithium battery. Patent Reference 2 describes a lithium battery having an electrode layer composed of sintered material including solid electrolyte material and metal oxide where electrically conductive particles are diversified. To form an electrode layer, metal oxide which is active material, solid electrolyte material, and electrically conductive particles are mixed with a weight ratio of 8:1:1. Solid electrolyte material is mixed so as to widen the contact area between active material and solid electrolyte material and lower the impedance in the electrode.
However, this technology has a problem that battery energy density is low because solid electrolyte material is included in the electrode layer and sufficient active material cannot be mixed in the electrode layer. Also sufficient amount of conductive particles cannot be mixed in the electrode layer by the same reason. Furthermore, since conductive particles are diversified in electrolyte material in the electrode, they contact with each other by point-contact or they do not contact at all, so that impedance reduction of the battery electrode cannot be effectively accomplished. This is also supported by the embodiment of Patent Reference 2 describing a battery having a collector layer to reduce impedance in addition to an electrode layer. This description shows that, by using the technology disclosed by Patent Reference 2, some minor reduction of impedance can be obtained by mixing conductive particles in the electrode, but satisfactory reduction of impedance cannot be obtained in order to fabricate high-performance batteries without using collector layers.
Accordingly, the present invention is intended to solve above problems and provide lithium ion secondary battery and manufacturing method thereof which enables to reduce electrode impedance, to simplify manufacturing process, and to reduce production cost without using collector layers in a multilayer all solid state lithium ion secondary battery including a laminate having a positive electrode layer, a solid electrolyte layer, and a negative electrode layer.